Renewable Cardanol-Based Surfactant Modified ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Renewable Cardanol-Based Surfactant Modified Layered Double Hydroxide as a Flame Retardant for Epoxy Resin Xin Wang, Ehsan Naderi Kalali, and De-Yi Wang* IMDEA Materials Institute, C/Eric Kandel, 2, Getafe 28906 Madrid, Spain S Supporting Information *

ABSTRACT: A biobased modifier (cardanol-BS) was successfully synthesized from renewable resource cardanol via the ring-opening of 1, 4-butane sultone (BS). Cardanol-BS modified layered double hydroxide (m-LDH) was developed through a one-step coprecipitation method and subsequently incorporated into epoxy resins (EPs) with different loadings using a combined technique of three-roll mill and ultrasonication. As a comparison, a pristine LDH/EP composite was also prepared using the same procedure. The XRD result indicated that the interlayer spacing of m-LDH was about 5fold enlarged compared with that of pristine LDH. As a result, the enlarged interlayer spacing of m-LDH facilitated the homogeneous dispersion of the nanoadditive in the epoxy matrix, as evidenced by TEM and XRD results. The flame retardant properties were improved with the increase of the m-LDH loading. With only 6 wt % m-LDH, the EP composite reached a limiting oxygen index (LOI) of 29.2% and UL-94 V0 rating. The peak heat release rate (PHRR), total heat release (THR), and total smoke production (TSP) values of EP/m-LDH-6% were decreased by 62%, 19%, and 45%, respectively, compared to those of pure EP. In contrast, pristine LDH did not show so high an efficiency as m-LDH in terms of the reduced PHRR, THR, and TSP, and also the EP/LDH-6% composite exhibited no rating in the UL-94 vertical burning test. These findings supported that the flame retardant behavior increased with improved dispersion of nanofiller in the polymer matrix. The well-dispersed m-LDH nanofillers were beneficial to improving the quality of char residue, which effectively inhibited flammable volatiles escaping from the interiors and served as an effective thermal insulation layer to shield the underlying matrix from the exterior heat irradiation. KEYWORDS: Cardanol, Layered double hydroxide, Epoxy resin, Morphology, Flame retardant mechanism

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INTRODUCTION Polymer/inorganic nanocomposites have showed great potential to match or exceed the mechanical, thermal, flame retardant performance of conventional composite filler even at relatively low filler loadings.1,2 However, enhancement efficiency depends strongly on the dispersion state of the nanofiller and the interfacial interaction between the filler and the matrix. In terms of maximal enhancement, homogeneous distribution of the nanofillers and strong interaction at the interface are favorable. At present, although nanocomposites employing various nanoscale fillers have been widely investigated, the intrinsic bundling of these nanofillers via van der Waals force still is a major challenge that hampers the development of polymer/ inorganic nanocomposites. Therefore, in order to improve the dispersion state of the nanofiller in the polymer matrix, modification of the nanofiller is necessary. Layered double hydroxide (LDH), also known as anionic clay, is a promising class of layered nanomaterial for preparing multifunctional polymer−matrix composites (PMCs).3 The general chemical formula of LDH can be illustrated as [M2+1−xM3+x (OH)2]An−x/n·mH2O, where M2+ is a divalent cation, M3+ is a trivalent cation, and An− is an interlayer anion.4 Because of the tunable species of M2+, M3+, and An− as well as © XXXX American Chemical Society

the value of x, the structure and properties of LDHs can be easily tailored. The layered structure of LDH can also be used as a versatile intercalation host that accommodates a wide variety of organic guest species. This procedure can be utilized for the modification of LDHs. Owing to these aforementioned fascinating properties, LDHs have received considerable research interest in the fields of wastewater treatment,5,6 drug delivery,7 energy application,8 and fabrication of polymer− matrix nanocomposites.9 Over the past few decades, LDH has been incorporated into polymers as a flame retardant nanoadditive because of its high water content, nontoxicity, and layered structure. The flame retardant effect of LDHs on various polymers, such as polyethylene-graf t-maleic anhydride,10 polypropylene,11 ethylene vinyl acetate,12 poly(methyl methacrylate),13 polystyrene,13 poly(vinyl alcohol), 14 polyvinyl chloride, 15 and epoxy resin,16−18 has been reported. However, its flame retardant efficiency strongly depends on the modification of LDH as the satisfactory dispersion of LDH in the polymer matrix is always Received: August 12, 2015 Revised: October 10, 2015

A

DOI: 10.1021/acssuschemeng.5b00871 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering difficult to achieve. Thus, selection of a modifier for LDH is quite important for achieving efficient flame retardant performance. On the basis of the discussion aforementioned, we developed here a biobased modifier (cardanol-BS) using cardanol from the renewable resource cashew nut shell liquid (CNSL), via ringopening of 1, 4-butane sultone (BS). Cardanol-BS modified layered double hydroxide (m-LDH) was incorporated into epoxy resins (EPs) with different loadings using a combined technique of three-roll mill and ultrasonication. The effect of mLDH on the thermal degradation and flame retardant behaviors of the resultant epoxy composites was investigated and compared to those of the epoxy composite containing unmodified LDH. The flame retardant mechanism was further proposed from the viewpoint of both condensed- and gaseousphases.

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Scheme 1. Diagrammatic Illustration of the Synthetic Route of Cardanol-BS Modified LDH

EXPERIMENTAL SECTION

Materials. Mg(NO3)2·6H2O, Al(NO3)3·9H2O, sodium hydroxide, 3-pentadecyl phenol, potassium tertiary butoxide, 1,4-butane sultone, and ethanol were supplied by Sigma-Aldrich Chemical Co. and used without further purification. Epoxy resin (Epoxydhedraz C) was purchased from R&G Faserverbundwerkstoffe GmbH-Germany. Diamino diphenyl sulfone (DDS) was purchased from TCI Chemicals Company. Deionized water is used for all experiments unless otherwise stated. Synthesis of 4-(3-Pentadecylphenoxy)butane-1-sulfonic Salt (Cardanol-BS). In a three-necked and round-bottomed flask equipped with a nitrogen inlet, a reflux condenser and a magnetic stirrer, 3petadecylphenol (0.10 mol), and potassium tertiary butoxide (0.20 mol) were dissolved in dry ethanol (100 mL). This solution was heated to 60 °C for 30 min under nitrogen atmosphere. After cooling down to room temperature, 1,4-butane sultone (0.20 mol) was added dropwise into the above solution. The reaction mixture was heated to reflux and allowed to stir for 40 h under a nitrogen atmosphere. It was cooled, and the white precipitate was isolated by suction filtration. The white potassium salt of the product was purified by passing through a silica gel column using methanol/chloroform (20/80 v/v) as eluent. Synthesis of Cardanol-BS Modified LDH. The 4-(3Pentadecylphenoxy)butane-1-sulfonic salt modified LDH was synthesized using the coprecipitation method. Typically, 4-(3pentadecylphenoxy)butane-1-sulfonic salt (0.10 mol) and deionized water (300 mL) were introduced into a 1000 mL three-necked round flask equipped with an iso-baric funnel, reflux-condenser, and pH meter and stirred until completely dissolved. Subsequently, an aqueous solution containing Mg(NO3)2·6H2O (0.2 mol) and Al(NO3)3·9H2O (0.1 mol) in deionized water (300 mL) was added into the above flask dropwise. The pH value of the mixture was kept at 10 ± 0.5 by 1 M NaOH aqueous solution. The resultant slurry was continuously stirred for 30 min; subsequently, it was allowed to age at 75 °C for 18 h. Finally, the resultant product was filtered, washed thoroughly with deionized water until pH 7, and then dried in an oven at 80 °C overnight until a constant weight was achieved. In addition, unmodified LDH was synthesized using a similar method without the addition of 4-(3-pentadecylphenoxy)butane-1-sulfonic salt. The synthetic route of cardanol-BS modified LDH is shown in Scheme 1. The chemical formula of the modified LDH can be represented as Mg2Al·(OH)6·(cardanol-BS)x·(NO3)1−x·2H2O, where x is the degree of intercalation of Cardanol-BS. On the basis of elemental analysis data of sulfur (4.3 wt % for modified LDH), the degree of intercalation of cardanol-BS can be calculated from the following equation:

S (wt%) =

Preparation of LDH/Epoxy Nanocomposites. A series of modified LDH/epoxy nanocomposites with different filler contents were prepared, and the formulations are listed in Table 1. In a typical

Table 1. Formulations of LDH/EP Composites formulations (wt %) samples

EP

m-LDH

pristine LDH

EP EP/m-LDH-1% EP/m-LDH-2% EP/m-LDH-4% EP/m-LDH-6% EP/LDH-6%

100 99 98 96 94 94

0 1 2 4 6 0

0 0 0 0 0 6

procedure of epoxy nanocomposites with 2 wt % modified LDH, modified LDH (2.0 g) was blended with epoxy resin (65.3 g) using a refined three-roll mill (EXAKT 80E, Germany) for 30 min; the mixture was diluted with acetone (50 mL), exposed to ultrasonication for 20 min at 60 °C, and then placed in a vacuum oven at 110 °C to remove the acetone solvent. Subsequently, DDS was added into the above mixture, stirred until DDS was totally dissolved, and degassed in a vacuum oven at 120 °C for 5 min. The mixture was then immediately poured into the preheated PTFE molds, and the curing procedure was set as 150 °C/1 h, 180 °C/2 h, and 200 °C/2 h. As a comparison, epoxy composites with 6 wt % unmodified LDH were prepared by the same procedure. Characterization and Instruments. 1H NMR spectrum was measured on a Bruker AVANCE-500 NMR spectrometer operating in the Fourier transform mode using DMSO-d6 as solvent. Fourier transform infrared spectra were recorded by a Nicolet iS50 FTIR spectrophotometer in KBr disc. The wavenumber range was set from 4000 to 400 cm−1, and the resolution was 4 cm−1. Wide-angle X-ray diffraction patterns of the samples were recorded on an XPERT-PRO X-ray diffractometer, using Cu Kα radiation (λ = 0.15418 nm) at 45 kV and 40 mA. Transmission electron microscopy (TEM) images were observed on a FEI TECNAI T20 microscope at an accelerated voltage of 200 kV. The nanocomposite samples were prepared using a Leica ultramicrotome to obtain 50 nm thick slices, and then the thin slices were transferred to the 200-mesh Cu grids for observation. Thermogravimetric analysis (TGA) of the samples was performed on a Q50 thermal analyzer (TA Instruments, USA) from 30 to 800 °C at a heating rate of 20 °C min−1 under nitrogen atmosphere. About 5.0 ± 0.05 mg of sample in powder for LDH and modified LDH or granule for epoxy composites was measured. Cone calorimeter tests

32x × 100% 275 + x(M w − 62)

where Mw is the molecular weight of cardanol-BS, 440. Therefore, the value of the degree of intercalation of cardanol-BS was 75%. B


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Figure 1. (a) 1H NMR spectrum of 4-(3-pentadecylphenoxy)butane-1-sulfonic salt; (b) FTIR spectra of cardanol-BS, pristine LDH, and cardanol-BS modified LDH; and (c) thermogravimetric analysis profiles of pristine LDH, cardanol-BS, and cardanol-BS modified LDH. were carried out on an FTT cone calorimeter, according to the procedures in ISO 5660-1. Each specimen with dimensions of 100 mm × 100 mm × 4 mm was irradiated horizontally at a heat flux of 50 kW m−2. The samples were mounted in aluminum foil and placed on a holder with the thermocouple under the sample. An infrared thermometer (Optris CTlaser MT-CF3) was used to monitor the surface temperature of the samples, and the system accuracy was ±1%. All measurements were repeated at least two times, and the results were averaged. Limiting oxygen index (LOI) measurements were conducted on an oxygen index model instrument (Fire Testing Technology, UK) according to ASTM D 2863-97 (the test specimen: 100 mm × 6.5 mm × 3.2 mm). The UL-94 vertical burning tests were performed on a burning chamber (Fire Testing Technology, UK) with the sample dimension of 127 mm × 12.7 mm × 4.0 mm according to the ASTM D 3801 standard. The morphology of the char residue after cone calorimetry tests was observed using a Zeiss EVO MA15 scanning electron microscope. All of the samples were sputter-coated with a conductive layer of gold prior to SEM observation. Thermogravimetric analysis-Fourier transform infrared spectrometry (TG-FTIR) was carried out on a Q50 thermal analyzer which was connected to the iS50 FT-IR spectrophotometer through a stainless steel transfer pipe. The pipe and gas cell were maintained at 300 and 250 °C, respectively, to avoid the condensation of the volatile products.

and 0.7 ppm corresponding to protons e, f, g, j, and k in the methylene and methyl groups originated from the 3-pentadecyl phenol unit. The resonance signals at 1.9 and 0.9 ppm are also attributed to the protons (labeled m and n) of the methylene group in the butane structure. Additionally, two resonance signals at 3.9 and 2.7 ppm (labeled l and h), assigned to the protons in the structure of Ar-O-CH2− and −CH2-SO3−, i m p l y i n g t h e s u c c e s s f u l sy n t h e s i s o f t h e 4 - ( 3 pentadecylphenoxy)butane- 1-sulfonic salt. Figure 1b shows the FTIR spectra of cardanol-BS, pristine LDH, and cardanol-BS modified LDH. In the FTIR spectrum of cardanol-BS, the broad absorption band at the range from 3150 to 3650 cm−1 is assigned to the physical absorbed water molecules; the strong absorption bands at 2930 and 2860 cm−1 are attributed to the stretching vibration of −CH3 and −CH2− groups, respectively; the characteristic peaks at 1605 and 1500 cm−1 are ascribed to the stretching of the benzene ring; the strong absorption bands at 1190 and 1060 cm−1 are due to the stretching vibration of sulfonate and ether groups, respectively, which also provide an evidence for the successful reaction between 3-pentadecyl phenol and 1, 4-butane sultone. In the FTIR spectrum of pristine LDH, several characteristic peaks are observed: the broad peak around 3490 cm−1 can be ascribed to the stretching of OH groups attached to Al and Mg ions in the layers;19 the small peak at 1620 cm−1 is assigned to the bending vibration of the interlayer water; and the strong band at 1385 cm−1 is owing to the asymmetric stretching of the carbonate anion. In contrast to the pristine LDH, some new peaks appear in the FTIR spectrum of cardanol-BS modified LDH. The appearance of the −CH3 and −CH2− stretching peaks (2930 and 2860 cm−1) together with the sulfonate stretching bands

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RESULTS AND DISCUSSION Structural Characterization. To validate the successful occurrence of the reaction between 3-pentadecyl phenol and 1,4-butane sultone, 1H NMR measurement is employed to characterize the product, as shown in Figure 1a. Composition of the 4-(3-pentadecylphenoxy) butane-1-sulfonic salt is determined by integration of 1H NMR signals at 7.2 and 6.5−6.8 ppm corresponding to protons a−d in the benzene ring, respectively, and the signals at 2.5, 1.7, 1.2−1.4, 0.9−1.0, C


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ACS Sustainable Chemistry & Engineering (1185 cm−1)20 confirms that cardanol-BS molecules have been intercalated into the interlayer space of MgAl-LDH. Thermogravimetric analysis (Figure 1c) was carried out to provide further evidence for successful modification of LDH. Pristine LDH starts to lose weight upon heating even below 200 °C, which is due to the removal of absorbed water, and the major mass loss occurs from 300 to 550 °C, presumably attributable to the dehydroxylation and the removal of interlayer anions.21 In contrast to pristine LDH, cardanol-BS modified LDH shows the different thermal degradation behaviors with much more mass loss at high temperature, which is probably attributed to the surface modification of LDH by cardanol-BS. Morphological Features. The morphological features of pristine LDH, cardanol-BS modified LDH, epoxy, and its composites were investigated using X-ray diffraction techniques, as shown in Figure 2. As can be seen, pristine LDH

2.1° was observed corresponding to an intergallery spacing of 4.20 nm. Except for this, no other visible reflection peaks were observed in the XRD patterns of EP/m-LDH composites, implying highly exfoliated and/or well intercalated LDH platelets within epoxy matrix. In contrast, the EP/LDH-6% composite shows several intense diffraction peaks at 2θ values of 9.9°, 19.8°, and 29.5°, which are similar to those of the pristine LDH, indicating the existence of the ordered structure of the pristine LDH in the epoxy matrix. TEM micrographs were taken to provide additional information in order to verify the results obtained from the XRD analysis as well as to directly observe the dispersion state of pristine LDH and m-LDH within the epoxy matrix. Figure 3 gives the TEM micrographs of EP/m-LDH and EP/LDH composites at different magnifications. The low magnification images provide information about the dispersion state, whereas the high magnification ones can distinguish whether intercalation and/or exfoliation have been obtained when LDHs are incorporated to epoxy matrices. In the case of EP/mLDH-1%, a low magnification TEM image (Figure 3a) reveals a typical feature for epoxy-based nanocomposites, a simultaneous presence of intercalated LDH plates, and small tactoids and stacks.22 Under high magnification (Figure 3e), it can be clearly seen that a well intercalated nanocomposite structure has been formed. The presence of the reflection peak at a 2θ value of 2.1° in the XRD pattern of EP/m-LDH-1% may thus be due to the formation of such intercalated structures. Similar TEM features were observed for EP/m-LDH-4% (Figure 3b,f) and EP/m-LDH-6% (Figure 3c,g). All of the EP/m-LDH nanocomposites appear to be of well intercalated structures, and the m-LDH nanoplatelets are randomly oriented in the epoxy matrix. In contrast, the low magnification image of EP/LDH6% (Figure 3d) shows that the dispersion of pristine LDH in the epoxy matrix is poor. Under high magnification (Figure 3h), the unmodified LDHs form large aggregates with thick stacking. These morphological features obtained from TEM and WAXD demonstrate that the modification of LDH by cardanol-BS is crucial to achieve the good dispersion state of LDH within the epoxy matrix. Thermal Properties. The influence of cardanol-BS modification on the thermal stability of the epoxy matrix was investigated by thermogravimetric analysis. Figure 4 gives the TGA profiles of EP, EP/m-LDH, and EP/LDH composites with different filler loadings. The relative thermal stability of the samples is evaluated by the temperature at 5% mass loss (T−5%) and the char residual percentage at 750 °C, as listed in Figure 4. It can be observed from Figure 4a that pure EP shows a single degradation stage ranging from 360 to 500 °C, corresponding to a T−5% of 395 °C. In contrast to neat EP, the EP/m-LDH composites exhibit a reduction in T−5%, which is probably attributed to the earlier decomposition of the thermally unstable additives (e.g., cardanol-BS in LDH). Furthermore, the addition of 1 wt % m-LDH improves the char yield to 13.2% for EP/m-LDH-1% from 11.9% for neat EP. The char yield is improved gradually as the m-LDH loading increases. In Figure 4b, it can be seen that EP/m-LDH-6% exhibits higher char yield than its counterpart containing unmodified LDH at the same filler loading, suggesting the enhanced char-formation ability by introducing cardanol-BS. Flame Retardant Properties. The flame resistant properties of the cured EP and its composites were studied by measuring their limiting oxygen index (LOI) and UL-94 vertical burning behaviors, as summarized in Table 2. Pure

Figure 2. WAXD profiles of pristine LDH, cardanol-BS modified LDH, epoxy, and its composites. The insert shows WAXD patterns in 2θ = 1−5° for EP/m-LDH composites.

exhibits the typical profile of LDH materials with sharp intense peaks at low θ values, while they become weaker and less defined at higher angular values. The characteristics bands at 2θ = 9.9°, 19.8°, and 29.5° are ascribed to the (003), (006), and (009) diffraction peaks, respectively, which are in good agreement with the previous literature. 10 From these parameters, the basal spacing of MgAl-LDH is estimated to be 0.88 nm according to the Bragg equation. The (003) basal reflection of cardanol-BS modified LDH shifts to 2θ = 2.3°, indicating that cardanol-BS anions have been intercalated into the interlayer galleries to give an increased interlayer spacing (d = 3.72 nm). Undoubtedly, the enlarged basal spacing facilitates the dispersion of LDH into the polymer matrix. The higher the gallery distance between LDH layers, the easier the macromolecules/monomers intercalate into the interlayer galleries. Thus, it is beneficial to obtaining uniformly dispersed LDH layers within a continuous polymer matrix. Additionally, no other crystalline phase is detected, implying the high purity of the product. Pure EP does not show any intensive Bragg diffraction peaks for 2θ values in the range of 1−40°, except for a broad and weak diffraction peak at a 2θ value of 17.3° owing to the amorphous phase of the epoxy matrix. In the case of EP/ m-LDH composites, a weak diffraction peak at a 2θ value of D


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Figure 3. TEM micrographs of EP/m-LDH-1% (a,e), EP/m-LDH-4% (b,f), EP/m-LDH-6% (c,g), and EP/LDH-6% (d,h) at different magnifications.

Table 2. Results of UL-94 and LOI Tests for Epoxy and Its Compositesa UL-94 samples

LOI (%)

t1/t2

rating

EP EP/m-LDH-1% EP/m-LDH-2% EP/m-LDH-4% EP/m-LDH-6% EP/LDH-6%

23.0 27.3 28.2 28.6 29.2 25.2

BC BC 6.3/BC 2.6/14.3 1.5/3.8 BC

NR NR NR V-1 V-0 NR

a Note: t1 and t2 are the average burning times after the first and the second exposures to the flame, respectively; BC, burns to clamp; NR, no rating.

LOI than EP/LDH-6%, indicating the superior flame retardant efficiency of modified LDH. With regard to the UL-94 vertical burning behavior, pure epoxy burns fiercely and exhibits no classification. Adding 1 or 2 wt % m-LDH into epoxy resins still indicates no rating in the UL-94 test. When the m-LDH loading is 4 wt %, the flame retardant grade is increased to V-1 rating. The great difference is observed in the EP/m-LDH-6% and the EP/LDH-6%: the former shows UL-94 V0 rating, whereas the latter shows no rating. In the case of EP/m-LDH-6%, the char can activate quickly after flame exposure, which extinguishes the flame propagation immediately. In contrast, EP/LDH-6% cannot form a thermally stable char layer so that the flame spreads fast along the sample (see the Video in Supporting Information). The fire behavior of pure EP and its composites containing m-LDH and LDH was investigated by means of a cone calorimeter. Figure 5 presents the heat release rate and the total heat release versus time curves of EP and its flame retardant composites. The detailed data are listed in Table 3. As can be seen, pure EP burns fiercely upon ignition, exhibiting a sharp peak heat release rate (PHRR) value of 790 kW m−2. The PHRR values of EP/m-LDH composites are reduced gradually as the m-LDH loading increases. The most efficient PHRR

Figure 4. TGA profiles of (a) EP and EP/m-LDH composites with different filler loadings; and (b) EP/m-LDH-6% and EP/LDH-6% composites.

epoxy shows a LOI value of 23.0%, suggesting an easily burning material. Incorporating 1 wt % m-LDH significantly increases the LOI of the epoxy composite to 27.3%, and further increasing the m-LDH loading improves the LOI value slightly. However, it is noted that EP/m-LDH-6% displays much higher E


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EP/m-LDH-2% shows features similar to those of EP/m-LDH1%. With the further increase of m-LDH loading, the residue becomes thicker. EP/m-LDH-6% shows the thickest char residue, which can shield the underlying materials more effectively. In contrast, the residue of EP/LDH-6% presents a loose structure with a lot of holes on the surface (Figure 6f). The microstructure of the char residue will be further investigated by SEM in the following section. From Table 3, it can be found that incorporation of m-LDH fillers has a slight influence on the time to ignition (TTI) of the EP composites. This earlier thermal degradation of EP composites is believed to be due to the char formation catalyzed by the m-LDH. The presence of the char layer shields the underlying matrix from being consumed by the flame. Therefore, the decreased thermal stability is likely essential rather than a drawback of this flame retardant system. With regard to the fire growth rate index (FIGRA) which is often equal to the ratio of PHRR and time to PHRR,24 all the EP composites containing m-LDH additives show decreased FIGRA compared to that of pure EP. The most striking result is observed in the case of EP/m-LDH-6%, a 75% reduction in FIGRA relative to pure EP, indicating the significantly lowered fire hazard quality of the material. As far as the TSP is concerned, all of the EP/m-LDH composites show decreased TSP compared to that of pure EP. This reduced smoke production can be explained by the fact that the presence of mLDH retains the EP molecules in the condensed phase instead of converting into the organic volatiles, as the organic volatiles are the major source of smoke particles.25 Furthermore, the addition of LDH and m-LDH leads to a reduction in the average effective heat of combustion (av-EHC), indicating the lower combustion heat generated from the burning of volatile gases (fuels) in gaseous-phase. Pure EP burnt completely, leaving little residue after the cone calorimeter test, whereas the residues of the EP/m-LDH composites increased with the increase of the filler loading. Because of the improvement of residual protection layers, the HRR and in particular the PHRR of the EP/m-LDH composites significantly decrease and shift the PHRR from the beginning toward the end of combustion. Therefore, the release amount of flammable gases decreases, and the total combustion time of the sample is prolonged.26 In comparison to EP/LDH-6%, the changes in the HRR curve are pronounced for EP/m-LDH-6%. EP/m-LDH-6% exhibits superior flame retardant properties over EP/LDH-6% in terms of lower PHRR, THR, FIGRA, and TSP. These superior flame retardant properties are attributed to the improved dispersion of the nanofillers as observed in TEM images, which have been also reported for other nanocomposites previously.27 Flame Retardant Mechanism. In order to verify the influence of cardanol-BS on the flame retardant properties of epoxy composites, the mechanism was investigated from both the condensed phase and the gaseous phase. TG-FTIR

Figure 5. (a) Heat release rate and (b) total heat release versus time curves of EP and its flame retardant composites.

reduction is observed in the case of EP/m-LDH-6%, an approximately 62% reduction in comparison to that of pure EP. Additionally, the characteristic shape of the heat release rate (HRR) curve changes from a typical one for noncharring materials (EP) to typical ones for charring materials (EP/mLDH composites).23 The total heat release versus time curves (Figure 5b) shows that pure EP releases heat very fast and that the total heat release reaches the maximum value (92.2 MJ m−2) after combustion. The addition of m-LDH inhibits the heat release, and the total heat release of EP/m-LDH composites decreased gradually with the increase of m-LDH loading. The reduction in the total heat release implies that more EP chains participate in the carbonization process, and therefore, less volatile products serve as “fuel” to feed back the flame. As a result, the total heat release (THR) values are dramatically reduced. Photos of the fire residue obtained in the cone calorimeter are displayed in Figure 6. EP burnt severely, leaving a very thin residue with big holes after the cone calorimeter test (Figure 6a). For EP/m-LDH-1%, the residue exhibits a cracked and porous surface with poor thermal resistance. The residue of

Table 3. Cone Calorimeter Data of EP and Its Composites under the Heat Flux of 50 kW m−2 samples EP EP/m-LDH-1% EP/m-LDH-2% EP/m-LDH-4% EP/m-LDH-6% EP/LDH-6%

TTI (s) 60 57 55 60 61 62

± ± ± ± ± ±

3 1 2 2 1 3

time to PHRR (s) 148 178 152 200 230 162

± ± ± ± ± ±

7 2 2 5 1 2

FIGRA (kW·(m−2 s−1)) 5.34 3.51 2.55 1.68 1.32 4.90 F

TSP (m2) 34.72 32.67 26.76 21.10 18.94 21.90

± ± ± ± ± ±

0.21 2.34 0.72 0.08 1.40 0.54

Av-EHC (MJ·kg−1)

residue (%)

± ± ± ± ± ±

8.1 ± 3.0 16.2 ± 1.4 19.7 ± 1.3 20.3 ± 1.1 23.2 ± 0.9 21.4 ± 1.2

19.20 18.52 17.45 17.18 16.41 18.34

0.18 0.37 0.23 0.45 0.35 0.23


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Figure 6. Residues at flameout of (a) EP and EP composites containing (b) 1.0 wt %, (c) 2.0 wt %, (d) 4.0 wt %, and (e) 6.0 wt % of m-LDH, and (f) EP/LDH-6%.

3100−2800 cm−1; bending vibration, 1253, 1340 cm−1), CO2 (2360 cm−1), aromatic compounds (1603 and 1505 cm−1), and C−O−C groups in ethers (1152 cm−1).25,28−30 To further understand the influence of m-LDH on the flame retardant properties of epoxy composites, the absorbance versus temperature curves of the selected degradation products for EP, EP/LDH-6%, and EP/m-LDH-6% composites are revealed in Figure 8. The degradation gaseous products can be mainly divided into two categories: one is the inflammable gases, such as water vapor and CO2; the other is the flammable gases, such as carbonyl, aromatic compounds and esters. The maximum absorbance intensity of the inflammable volatiles for EP/LDH-6% and EP/m-LDH-6% composites is much higher than that for pure EP, which can dilute the concentration of the flammable volatiles. Meanwhile, the maximum absorbance intensity of the flammable volatiles for EP/m-LDH-6% composite is much lower compared to those for pure EP and EP/LDH-6% composite. The decreased amount of the carbonyl, aromatic, and ether compounds implies less “fuel” to be fed back to the flame, resulting in the reduced heat release rate and total heat release observed in cone calorimeter tests. In comparison to the EP/LDH-6% composite, the lower amount of the flammable volatiles of the EP/m-LDH-6% composite is attributed to the superior barrier effect with the uniform dispersion of the nanofillers. The morphology of the postburn samples after cone calorimeter tests was investigated using SEM, as shown in Figure 9. As can be observed, the pure EP (Figure 9a) exhibits a loose and porous residue after burning, indicative of its high flammability. The heat and oxygen easily penetrate through these holes and gaps, and thus, the control sample burns fiercely. In the case of the EP/LDH-6% composite (Figure 9b), the residue shows a cracked surface. Such a broken char layer still cannot effectively protect the underlying matrix from the attack by flame. In contrast, the EP/m-LDH-6% composite (Figure 9c) shows a compact and continuous residue,

technique was utilized to study the thermal decomposition of volatiles during combustion. The TG-FTIR results reported here have been normalized by the mass of sample. The normalized FTIR spectra of the pyrolysis gaseous products of EP, EP/LDH-6%, and EP/m-LDH-6% composites at the maximum degradation rate are presented in Figure 7. The

Figure 7. FTIR spectra of the pyrolysis gaseous products of (a) EP, (b) EP/LDH-6%, and (c) EP/m-LDH-6% composites at the maximum degradation rate.

spectra of both EP/LDH-6% and EP/m-LDH-6% composites do not differ significantly from that of EP, meaning that the addition of m-LDH or LDH has a slight influence on the decomposition volatiles of EP during combustion. Some small molecular gaseous products evolved from EP, EP/LDH-6%, and EP/m-LDH-6% composites are identified unambiguously by the characteristic FTIR signals, such as C−H groups in unsaturated and saturated hydrocarbons (stretching vibration, G


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Figure 8. Absorbance versus temperature curves of typical decomposition products for EP, EP/LDH-6%, and EP/m-LDH-6% composites.

Figure 9. SEM micrographs of char residues for (a) EP, (b) EP/LDH-6%, and (c) EP/m-LDH-6% composites.

answering for the significant reduction in PHRR. According to a previous report,31−33 the quality of residue correlates to the dispersion state of the nanofillers. The well dispersed layered morphology of m-LDH is favorable in terms of forming an effective protection layer during combustion. In order to confirm the thermal insulating effect of the char residues, the temperatures of the surface and that beneath the samples have been measured by the thermocouples in real time during the cone calorimeter tests.34−36 In this work, the temperature versus time curves of pure EP, EP/LDH-6%, and EP/m-LDH-6% composites are plotted in Figure 10. The temperature on the surfaces of all of the samples is nearly the same, showing a sharply increased temperature upon the ignition and stabilizing at approximately 680 °C during the combustion. In the case of pure EP, the temperature beneath the sample increases quickly and is close to the surface temperature beyond 200 s, indicating the poor thermal insulating effect of the char residue. The incorporation of pristine LDH slows down the increased trend of the temperature beneath EP/LDH-6% compared to that of pure EP but still cannot inhibit the temperature increasing to more

Figure 10. In situ temperature versus time curves of pure EP, EP/ LDH-6%, and EP/m-LDH-6% composites detected in cone calorimetry tests.

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Figure 11. Schematic illustration of the flame retardant mechanism of cardanol-BS modified LDH in EP composites.

than 600 °C. In contrast, the temperature beneath EP/m-LDH composites increases slowly, and furthermore, the temperature beneath the sample is gradually lowered with the increase of mLDH loading. In the case of EP/m-LDH-6%, the temperature is always below 400 °C during the whole burning process, demonstrating that the better quality char shows more efficiency in thermal insulation effect. Upon the basis of the aforementioned fire behavior, the flame retardant mechanism is proposed, as illustrated in Figure 11. The quality of the char residue correlates close to the dispersion state of the nanofiller in the polymer matrix. The pristine LDH is apt to form aggregated stacks in the epoxy matrix, and thus, the residue shows a cracked surface without sufficient cohesion. The degradation volatiles can escape from these cracks freely to feed the flame, and meanwhile, the heat irradiation easily penetrates through these gaps to consume the underlying polymer matrix. In contrast, the well-dispersed mLDH leads to a compact and continuous residue, which can serve as an excellent insulator (protect the underlying matrix from destruction by the exterior heat irradiation) and mass transport barrier simultaneously (inhibit the flammable gases escaping from the interior).32 Both the barrier and thermal insulation effect are important to improve the fire resistance of epoxy composites. These findings correspond well with the superior flame resistance of EP/m-LDH-6% over EP/LDH-6% regarding the much lower PHRR, THR, TSP, of EP/m-LDH6% as well as the great difference between EP/m-LDH-6% (V0 rating) and EP/LDH-6% (no rating) in UL-94 vertical burning tests.

showed no rating in the UL-94 vertical burning test. Furthermore, the EP/m-LDH-6% composite also exhibited significantly lowered PHRR, THR, and TSP compared to EP/ LDH-6% composite observed from cone calorimeter tests. The superior flame retardant properties of EP/m-LDH over EP/ LDH were probably attributed to the well-dispersed m-LDH nanofillers that led to a compact and continuous residue during combustion, which served as an effective thermal insulation layer to retard flammable volatiles escaping from the interior and also shield the underlying matrix from exterior heat irradiation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00871.

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UL-94 vertical burning test of EP/m-LDH-6% (AVI) UL-94 vertical burning test of EP/LDH-6% (AVI)

AUTHOR INFORMATION

Corresponding Author

*Tel: +34 91 549 34 22. Fax: +34 91 550 30 47. E-mail: deyi. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partly funded by the European Commission under the seventh Framework Programme (Marie Curie Career Integration Grant, GA-321951) and Spanish Ministry of Economy and Competitiveness (MINECO) under Ramón y Cajal grant (RYC-2012-10737) and COMETAD project (MAT2014-60435-C2-2-R).

CONCLUSIONS A biobased surfactant, cardanol-BS, was successfully synthesized and used to modify a layered double hydroxide nanofiller. A series of EP/m-LDH nanocomposites with different filler loadings were prepared through mechanical shear and ultrasonication. As a comparison, pristine LDH/EP composite was also prepared using the same procedure. TEM and XRD results showed that cardanol-BS modified LDH possessed a uniform dispersion state compared to that of pristine LDH due to the enlarged interlayer spacing of the m-LDH. The addition of mLDH was proved to be efficient in improving the flame retardant properties of epoxy resins. At a relatively low loading (6 wt %) of m-LDH, the EP composite reached a LOI of 29.2% and UL-94 V0 rating. In contrast, the EP/LDH-6% composite

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